Corrosion Resistant Metallurgical Powder Compositions

ABSTRACT

Provided are corrosion resistant metallurgical powder compositions, corrosion resistant compacted articles prepared from metallurgical powder compositions, and methods of preparing the same. Corrosion resistant metallurgical powder compositions include as a major component, an iron-based powder and, as a minor component, alloy additives that include chromium, and carbon. Upon compaction and sintering, the iron-based powder and alloy additives form a dual phase alloy system. The dual phase alloy system is denoted by an admixed martensite and ferrite microstructure. This unique microstructure results in beneficial physical properties, such as for example, high strength, hardness, and ductility, impact energy, and fatigue endurance, while maintaining resistance to corrosion.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser. No. 11/385,292, filed Mar. 21, 2006, which claims the benefit of U.S. Provisional Application No. 60/692,387, filed Jun. 20, 2005, the entireties of which are incorporated herein.

FIELD OF THE INVENTION

The present invention relates to metallurgical powder compositions, compacted articles prepared from metallurgical powder compositions, and methods of making the same. More particularly, the present invention is directed to corrosion resistant metallurgical powder compositions, corrosion resistant compacted articles prepared from metallurgical powder compositions, and methods of preparing the same.

BACKGROUND OF THE INVENTION

Iron-based particles have long been used as a base material in the manufacture of structural components by powder metallurgical techniques. These techniques involve first compacting iron-based particles in a die under high pressures in order to produce an article having a desired shape. After the compacting step, the structural component may undergo a sintering step to impart additional strength.

Research in the powder metallurgical manufacture of compacted components using iron-based powders has been directed to the development of iron powder compositions that enhance certain physical properties without detrimentally affecting other properties. Desired properties that often must be balanced include, for example, high density, strength, and corrosion resistance.

Conventional iron based powders having corrosion resistance properties, like stainless steels, have been use for example in the automobile industry. The automobile industry generally seeks to maintain the cost advantage of powder metallurgy techniques while maintaining requisite physical characteristics, including for example, corrosion resistance and strength, especially strength at high temperatures. However, the strength of some stainless steels generally can not be improved without increasing the alloy content of a composition or utilizing secondary processing techniques, both of which increase the cost of preparing compacted articles. Although some improvement in strength is observed by increasing the alloy content of a compositions, the improvement is often insufficient to enable the use of some stainless steels in high strength applications. Moreover, traditional secondary processing techniques utilized to increase strength are unavailable for certain stainless steels. For example, ferritic stainless steels are generally not heat treated because heating and fast cooling do not result in transformations that increase strength or hardness.

Accordingly, there exists a need for high strength, low cost stainless steels with beneficial corrosion resistance properties.

SUMMARY OF THE INVENTION

Provided are corrosion resistant metallurgical powder compositions, corrosion resistant compacted articles prepared from metallurgical powder compositions, and methods of preparing the same. Corrosion resistant metallurgical powder compositions include as a major component, an iron-based powder and, as a minor component, alloy additives that include chromium, and carbon. Upon compaction and sintering, the iron-based powder and alloy additives form a dual phase alloy system. The dual phase alloy system is characterized by an admixed martensite and ferrite microstructure. This unique microstructure results in beneficial physical properties, such as for example, high strength, hardness, and ductility, impact energy, and fatigue endurance, while maintaining resistance to corrosion.

Corrosion resistant metallurgical powder compositions are low cost alternatives to conventional alloys, which require high alloy content or secondary processing steps, e.g., heat treatments, to provide sufficient strength. Thus, corrosion resistant metallurgical powder compositions are an improved high-strength, corrosion-resistant stainless steel product as compared with presently-existing compositions having similar chromium levels.

Corrosion resistant metallurgical powder compositions may include alloying additives, such as for example, molybdenum, copper, nickel, sulfur, phosphorus, silicon, manganese, titanium, aluminum, or combinations thereof. The alloying additives may be added as discrete elemental powders, or they may be prealloyed with one or more metal powders, e.g., iron powders.

One may use a ferritic factor, K_(m), to define the alloy system of corrosion resistant metallurgical powder compositions. Corrosion resistant metallurgical powder composition have a ferritic factor of from about 6 to about 20. “Ferritic factor” is defined by the formula:

K _(m)=(Wt. % Cr)−40(Wt. % C+Wt. % N)+4(Wt. % Mo)−5(Wt. % Cu)−4(Wt. % Ni)−20(Wt. % P)+6(Wt. % Si)−2(Wt. % Mn)+8(Wt. % Ti)+2(Wt. % Al)

Methods of preparing compacted articles include the steps of providing a corrosion resistant metallurgical powder composition, and compacting the composition to form a green compact. Then, the green compact is sintered to form the dual phase microstructure. Upon sintering, the present compositions provide unusually high strength and high fatigue endurance limits while maintaining ductility and high impact strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph of an exemplary corrosion resistant compacted article.

FIG. 2A is an exemplary pseudo phase diagram for corrosion resistant metallurgical powder compositions.

FIG. 2B is another exemplary pseudo phase diagram for corrosion resistant metallurgical powder compositions.

FIG. 3 is a graph of sintered density verses ferritic factor for exemplary sintered corrosion resistant compacted articles and conventional stainless steel compacts.

FIG. 4 is a graph of transverse rupture strength verses ferritic factor for exemplary sintered corrosion resistant compacted articles and conventional stainless steel compacts.

FIG. 5 is a graph of hardness verses ferritic factor for exemplary sintered corrosion resistant compacted articles and conventional stainless steel compacts.

FIG. 6 is a graph of ultimate tensile strength and yield strength verses ferritic factor for exemplary sintered corrosion resistant compacted articles and conventional stainless steel compacts.

FIG. 7 is a graph of elongation verses ferritic factor for exemplary sintered corrosion resistant compacted articles and conventional stainless steel compacts.

FIG. 8 is a graph of volume percent of ferrite verses ferritic factor for corrosion resistant compacted articles.

FIG. 9 is a graph of fatigue endurance limits (KSI) verses Tensile Strength (KSI) for corrosion resistant compacted articles.

FIG. 10 is a micrograph of a conventional stainless steel compact prepared from commercially available 410L grade powder with graphite addition.

FIG. 11 is a micrograph of a conventional stainless steel compact prepared from commercially available 17-4PH grade powder.

FIG. 12 is a micrograph of a conventional stainless steel compact prepared from commercially available 409LNi grade powder.

FIG. 13 is a micrograph of an exemplary corrosion resistant compact prepared from a corrosion resistant metallurgical powder composition of the present invention.

FIG. 14 is an exemplary micrograph of a conventional rolled plate wrought stainless steel.

FIG. 15 is a graph of percent mass gain verses number of oxidation cycles for corrosion resistant compacts and conventional stainless steel compacts.

FIG. 16 is a graph of green density verses compaction pressure for an exemplary corrosion resistant compact.

FIG. 17 is a graph of green strength verses compaction pressure for an exemplary corrosion resistant compact.

FIG. 18 is a graph of sintered density verses compaction pressure for three exemplary corrosion resistant compact that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 19 is a graph of dimensional change verses compaction pressure for three exemplary corrosion resistant compacts that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 20 is a graph of tensile strength verses compaction pressure for three exemplary corrosion resistant compacts that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 21 is a graph of yield strength verses compaction pressure for three exemplary corrosion resistant compacts that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 22 is a graph of tensile elongation verses compaction pressure for three exemplary corrosion resistant compacts that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 23 is a graph of impact energy verses compaction pressure for three exemplary corrosion resistant compacts that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 24 is a graph of transverse rupture strength verses compaction pressure for three exemplary corrosion resistant compacts that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 25 is a graph of apparent hardness verses compaction pressure for three exemplary corrosion resistant compacts that were sintered at 2300° F., 2200° F., and 2050° F. respectively.

FIG. 26 is a micrograph of an exemplary etched microstructure of a corrosion resistant compact.

FIG. 27 is another micrograph of an exemplary etched microstructure of a corrosion resistant compact.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Provided are corrosion resistant metallurgical powder compositions, corrosion resistant compacted articles prepared from metallurgical powder compositions, and methods of preparing the same. Corrosion resistant metallurgical powder composition include as a major component, an iron-based powder and, as a minor component, alloy additives that include chromium, and carbon. Upon compaction and sintering, the iron-based powder and alloy additives form a dual phase alloy system. The dual phase alloy system is denoted by an admixed martensite and ferrite microstructure. This unique microstructure results in beneficial physical properties, such as for example, high strength, hardness, and ductility, impact energy, and fatigue endurance, while simultaneously maintaining resistance to corrosion.

Corrosion resistant metallurgical powder compositions are low cost alternatives to conventional alloys, which require high alloy content or secondary processing steps, e.g., heat treatments, to provide high strength compacted parts. Thus, corrosion resistant metallurgical powder compositions are an improved high-strength, corrosion-resistant stainless steel product as compared with presently-existing compositions of similar chromium levels.

Corrosion-resistant metallurgical powder compositions are composed of, as a major component, an iron-based powder, and, as a minor component, alloy additive powders. Iron based powders, as that term is used herein, are powders of pure iron, substantially pure iron, powders of iron prealloyed with alloying elements, such as for example, steel-producing elements, and powders of iron to which such other alloying elements have been coated or diffusion bonded. Iron based powders may be an admixture of an atomized iron powder and a sponge iron, or other type of iron powder. Iron based powders may be atomized by conventional water atomization or gas atomization techniques commonly known to those skilled in the art. Preferably, iron based powders are water atomized iron based powders.

When ranges are used herein for physical or chemical properties, such as for example alloy content, all combinations and subcombinations of ranges and specific embodiments therein are intended to be included.

Substantially pure iron powders are powders of iron containing not more than about 1.0% by weight, preferably no more than about 0.5% by weight, of normal impurities. Examples of such highly compressible, metallurgical-grade iron powders are the ANCORSTEEL 1000 series of pure iron powders, e.g. 1000, 1000B, and 1000C, available from Hoeganaes Corporation, Riverton, N.J. For example, ANCORSTEEL 1000 iron powder, has a typical screen profile of about 22% by weight of the particles below a No. 325 sieve (U.S. series) and about 10% by weight of the particles larger than a No. 100 sieve with the remainder between these two sizes (trace amounts larger than No. 60 sieve). The ANCORSTEEL 1000 powder has an apparent density of from about 2.85-3.00 g/cm³, typically 2.94 g/cm³. Other substantially pure iron powders that can be used in the invention are typical sponge iron powders, such as Hoeganaes' ANCOR MH-100 powder.

Exemplary prealloyed iron-based powders are stainless steel powders. These stainless steel powders that are commercially available in various grades in the Hoeganaes ANCOR® series, such as the ANCOR® 303L, 304L, 316L, 410L, 430L, 434L, and 409Cb powders. Also, iron-based powders include tool steels made by powder metallurgy methods.

Other exemplary iron-based powders are substantially pure iron powders prealloyed with alloying elements, such as for example molybdenum (Mo). Iron powders prealloyed with molybdenum are produced by atomizing a melt of substantially pure iron containing from about 0.5 to about 2.5 weight percent Mo. An example of such a powder is Hoeganaes' ANCORSTEEL 85HP steel powder, which contains about 0.85 weight percent Mo, less than about 0.4 weight percent, in total, of such other materials as manganese, chromium, silicon, copper, nickel, molybdenum or aluminum, and less than about 0.02 weight percent carbon. Other examples of molybdenum containing iron based powders are Hoeganaes' ANCORSTEEL 737 powder (containing about 1.4 wt. % Ni—about 1.25 wt. % Mo—about 0.4 wt. % Mn; balance Fe), ANCORSTEEL 2000 powder (containing about 0.46 wt. % Ni—about 0.61 wt. % Mo—about 0.25 wt. % Mn; balance Fe), ANCORSTEEL 4300 powder (about 1.0 wt. % Cr—about 1.0 wt. % Ni—about 0.8 wt. % Mo—about 0.6 wt. % Si—about 0.1 wt. % Mn; balance Fe), and ANCORSTEEL 4600V powder (about 1.83 wt. % Ni—about 0.56 wt. % Mo—about 0.15 wt. % Mn; balance Fe). Other exemplary iron-based powders are disclosed in U.S. application Ser. No. 10/818,782, which is herein incorporated by reference in its entirety.

An additional pre-alloyed iron-based powder is disclosed in U.S. Pat. No. 5,108,493, which is herein incorporated by reference in its entirety. These steel powder compositions are an admixture of two different pre-alloyed iron-based powders, one being a pre-alloy of iron with 0.5-2.5 weight percent molybdenum, the other being a pre-alloy of iron with carbon and with at least about 25 weight percent of a transition element component, wherein this component comprises at least one element selected from the group consisting of chromium, manganese, vanadium, and columbium. The admixture is in proportions that provide at least about 0.05 weight percent of the transition element component to the steel powder composition. An example of such a powder is commercially available as Hoeganaes' ANCORSTEEL 41 AB steel powder, which contains about 0.85 weight percent molybdenum, about 1 weight percent nickel, about 0.9 weight percent manganese, about 0.75 weight percent chromium, and about 0.5 weight percent carbon.

A further example of iron-based powders are diffusion-bonded iron-based powders which are particles of substantially pure iron that have a layer or coating of one or more other alloying elements or metals, such as steel-producing elements, diffused into their outer surfaces. A typical process for making such powders is to atomize a melt of iron and then combine this atomized powder with the alloying powders and anneal this powder mixture in a furnace. Such commercially available powders include DISTALOY 4600A diffusion bonded powder from Hoeganaes Corporation, which contains about 1.8% nickel, about 0.55% molybdenum, and about 1.6% copper, and DISTALOY 4800A diffusion bonded powder from Hoeganaes Corporation, which contains about 4.05% nickel, about 0.55% molybdenum, and about 1.6% copper.

Other iron-based powders that are useful in the practice of the invention are ferromagnetic powders. An example is a powder of iron pre-alloyed with small amounts of phosphorus.

The particles of iron-based powders, such as the substantially pure iron, diffusion bonded iron, and pre-alloyed iron, have a distribution of particle sizes. Typically, these powders are such that at least about 90% by weight of the powder sample can pass through a No. 45 sieve (U.S. series), and more preferably at least about 90% by weight of the powder sample can pass through a No. 60 sieve. These powders typically have at least about 50% by weight of the powder passing through a No. 70 sieve and retained above or larger than a No. 400 sieve, more preferably at least about 50% by weight of the powder passing through a No. 70 sieve and retained above or larger than a No. 325 sieve. Also, these powders typically have at least about 5 weight percent, more commonly at least about 10 weight percent, and generally at least about 15 weight percent of the particles passing through a No. 325 sieve. Reference is made to MPIF Standard 05 for sieve analysis.

As such, metallurgical powder compositions can have a weight average particle size as small as one micron or below, or up to about 850-1,000 microns, but generally the particles will have a weight average particle size in the range of about 10-500 microns. Preferred are iron or pre-alloyed iron particles having a maximum weight average particle size up to about 350 microns; more preferably the particles will have a weight average particle size in the range of about 25-150. In a preferred embodiment, metallurgical powder compositions have a typical particle size of less than 150 microns (−100 mesh), including, for example, powders having 38% to 48% of particles with a particle size of less than 45 microns (−325 mesh).

The described iron-based powders that constitute the base-metal powder, or at least a major amount thereof, are preferably atomized powders. These iron-based powders have apparent densities of at least 2.75, preferably between 2.75 and 4.6, more preferably between 2.8 and 4.0, and in some cases more preferably between 2.8 and 3.5 g/cm³.

Iron-based powders constitute a major portion of the metallurgical powder composition, and generally constitute at least about 70 weight percent, preferably at least about 80 weight percent, and more preferably at least about 85 weight percent of the metallurgical powder composition.

Corrosion resistant metallurgical powder compositions incorporate one or more alloying additives that enhance the mechanical or other properties of final compacted parts. Alloying additives are combined with the base iron powder by conventional powder metallurgy techniques known to those skilled in the art, such as for example, blending techniques, prealloying techniques, or diffusion bonding techniques. Preferably, alloy additives are combined with an iron-based powder by prealloying techniques, i.e., preparing a melt of iron and the desired alloying elements, and then atomizing the melt, whereby the atomized droplets form the powder upon solidification.

Alloying additives are those known in the powder metallurgical industry to enhance the corrosion resistance, strength, hardenability, or other desirable properties of compacted articles. Steel-producing elements are among the best known of these materials. Examples of alloying elements include, but are not limited to, chromium, graphite (carbon), molybdenum, copper, nickel, sulfur, phosphorus, silicon, manganese, titanium, aluminum, magnesium, gold, vanadium, columbium (niobium), or combinations thereof. Preferred alloying elements are steel producing alloys, such as for example, chromium, graphite, molybdenum, nickel, or combinations thereof. The amount of the alloying element or elements incorporated depends upon the properties desired in the final metal part. Pre-alloyed iron powders that incorporate such alloying elements are available from Hoeganaes Corp. as part of its ANCORSTEEL line of powders.

The unique challenges presented by powder metallurgy techniques precludes direct analogy and correlation between wrought steel and powder metallurgy processes. For example, wrought steel compositions and processes do not provide the advantages associated with powder metallurgical compositions and process, which include, inter alia, production to near net shape, few or no required secondary operations, high material utilization, superior homogeneity, availability of unique compositions and structures, and ability to form fine and isotropic metallurgical structures.

Corrosion resistant metallurgical powder compositions are described in, for example, C. Schade et. al., “Development Of A High Strength Dual Phase P/M Stainless Steel” Powder Metallurgy & Particulate Materials, 2005. Moreover, conventional powder metallurgy and wrought steel compositions and processes are described, for example, in U.S. Pat. Nos. 5,976,216 to Samal et. al., 5,856,625 to Saunders et. al., 5,681,528 to Martin et. al., 4,964,926 to Hill et. al., 4,881,991 to Beeton et. al., 4,608,099 to Davidson et. al., the contents of each of which is herein incorporated by reference in its entirety. Additional conventional compositions and processes are described in, for example, S. Shah, J. McMillen, P. Samal, & L. Pease “Mechanical Properties Of High Temperature Sintered P/M 409LE And 409LNi Stainless Steels Utilized In The Manufacturing Of Exhaust Flanges And Oxygen Sensor Bosses,” SAE Technical Paper Series, 2003-01-0451, reprinted from 42 Volt Technology 2003; R. Knutsen & R. Hutchings, “Occurrence Of Non-Metallic Inclusions In 3CR12Steel And Their Effect On Impact Toughness,” Materials Science and Technology, Vol. 4, p. 127, February 1988; R. Kaltenhauser, “Improving The Engineering Properties Of Ferritic Stainless Steels,” Metals Engineering Quarterly, p. 41 May 1971; A. Ball & J. Hoffman, “Microstructure And Properties Of A Steel Containing 12% Cr,” Metals Technology, p. 329, September 1981; R. Knutsen, “Influence Of Compositional Banding On Grain Anisotropy In 3CR12Steel,” Materials Science and Technology, Vol. 8, p. 621, July 1992; and F. Fletcher, B. Ferry, & D. Beblo, “High Performance Corrosion-Resistant Structural Steel,” International Symposium on High Performance Steels for Structural Applications, 1995, the contents of each of which is herein incorporated by reference in its entirety.

Corrosion resistant metallurgical powders may include any concentration of carbon, sulfur, oxygen and nitrogen. For example, some embodiments may require high concentrations of carbon, and nitrogen to promote the formation of high temperature martensite. Nitrogen concentrations, in particular, stabilize the martensite phase of a dual phase microstructure. But, carbon, sulfur, oxygen, and nitrogen additives are preferably kept as low as possible in order to improve compressibility and sinterability. Preferably, corrosion resistant metallurgical powder compositions contain, independently, from about 0.001 to about 0.1 weight percent carbon, about 0.0 to about 0.1 weight percent sulfur, about 0.0 to about 0.3 weight percent oxygen, and about 0.0 to about 0.1 weight percent nitrogen. More preferably, corrosion resistant metallurgical powder compositions contain, independently, from about 0.001 to about 0.1 weight percent carbon, about 0.0 to about 0.1 weight percent sulfur, about 0.0 to about 0.1 weight percent oxygen, about 0.0 to about 0.1 weight percent nitrogen.

Similarly, corrosion resistant metallurgical powders may include silicon additions in any concentration. However, high silicon concentrations, for example greater than about 0.85 weight percent, are utilized to produce a powder that is low in oxygen. Typically, the silicon level in a melt is increased prior to atomization. Silicon additions add strength to compacted parts, and also stabilize the ferrite phase of the dual phase microstructure. Preferably, corrosion resistant metallurgical powder compositions contain up to about 1.5 weight percent silicon. More preferably, corrosion resistant metallurgical powder compositions contain from about 0.1 to about 1.5 weight percent silicon, and even more preferably from about 0.85 to about 1.5 weight percent silicon.

Corrosion resistant metallurgical powders may contain chromium in any concentration. Chromium additions stabilize the ferritic phase of the dual phase microstructure and impart corrosion resistance. Generally, chromium additions also impart strength, hardenability, and wear resistance. Preferably, corrosion resistant metallurgical powder compositions contain from about 5.0 to about 30.0 weight percent chromium. More preferably, corrosion resistant metallurgical powder compositions contain from about 10 to about 30.0 weight percent chromium, and even more preferably from about 10 to about 20 weight percent chromium.

Corrosion resistant metallurgical powders may contain nickel in any concentration. Nickel is generally used to promote the formation of high temperature martensite. In addition, nickel improves toughness, impact resistance and corrosion resistance. Although nickel additions may reduce compressibility at high concentrations, nickel may be used at moderate levels without dramatically decreasing compressibility. Preferably, corrosion resistant metallurgical powder compositions contain up to about 2.0 weight percent nickel. More preferably, corrosion resistant metallurgical powder compositions contain from about 0.1 to about 1.5 weight percent nickel, and even more preferably from about 1.0 to about 1.5 weight percent nickel.

Corrosion resistant metallurgical powders may contain manganese in any concentration. Manganese additions increase the work hardening capacity of compacted parts and promote the formation of high temperature martensite. However, manganese concentration is generally kept at low levels because it contributes to the formation of porous oxides on the surface of powders. This porous oxide increases oxygen concentrations on powder surface, which impedes sintering. Typically, manganese additions also decrease the compressibilty of powders. Preferably, corrosion resistant metallurgical powder compositions contain up to about 0.5 weight percent manganese. More preferably, corrosion resistant metallurgical powder compositions contain from about 0.01 to about 0.5 weight percent manganese, and even more preferably from about 0.1 to about 0.25 weight percent manganese.

Corrosion resistant metallurgical powders may contain copper in any concentration. Copper additions increase corrosion resistance, while also providing solid solution strengthening. Although copper additions may reduce compressibility at high concentrations, copper may be used at moderate levels without dramatically decreasing compressibility. Copper additions also promote the formation of high temperature martensite. Preferably, corrosion resistant metallurgical powder compositions contain from about 0.01 to about 1.0 weight percent copper. More preferably, corrosion resistant metallurgical powder compositions contain from about 0.1 to about 0.8 weight percent copper, and even more preferably from about 0.25 to about 0.75 weight percent copper.

Corrosion resistant metallurgical powders may contain molybdenum in any concentration. Molybdenum additives increase hardenability, high temperature strength, and impact toughness while contributing to high-temperature oxidation resistance. Molybdenum also contributes to the stabilization of the ferritic phase of the dual phase microstructure of compacted parts. Preferably, corrosion resistant metallurgical powder compositions contain from about 0.01 to about 1.0 weight percent molybdenum. More preferably, corrosion resistant metallurgical powder compositions contain from about 0.5 to about 1.0 weight percent molybdenum, and even more preferably from about 0.85 to about 1.0 weight percent molybdenum.

Corrosion resistant metallurgical powders may contain titanium and aluminum in any concentration. Titanium and aluminum additives, individually, stabilize the ferrite phase of the dual phase microstructure. Preferably, corrosion resistant metallurgical powder compositions contain up to about 0.2 weight percent titanium and, independently, up to about 0.1 weight percent aluminum.

Corrosion resistant metallurgical powders may contain phosphorus in any concentration. Phosphorus additives promote the formation of high temperature martensite. Preferably, corrosion resistant metallurgical powder compositions contain up to about 0.1 weight percent phosphorus.

Alloy additives are selected to form an alloy system that provides desired properties. The selection of individual alloy elements and the amounts thereof should be chosen so as not to pose a significantly detriment to the physical properties of the composition. For example, elements such as nickel, molybdenum, and copper may be added in relatively small proportions to increase green density.

Corrosion resistant metallurgical powders, such as for example, stainless steels can be classified in a variety of ways. The key differences in properties, however, are determined by the type of alloy matrix created after processing. Alloys systems are based predominantly around ferritic, austenitic, and martensitic alloy matrices.

Conventional ferritic stainless steels are ferrous based alloys generally containing additions of chromium with relatively low concentrations of carbon. Typically, ferritic stainless steels contain from about 10.5 to about 27.0 weight percent chromium. Conventional ferritic stainless steels optionally also contain nickel additions. These alloys exhibit good corrosion resistance especially at higher chromium levels (superferritic) with a reduced tendency to the crevice type corrosion found in austenitic stainless steels. However, the ferritic type matrix is comparatively soft and has a relatively low work hardening response. Consequently, conventional ferritic alloys usually exhibit poor wear characteristics.

Conventional martensitic stainless steels are generally ferrous alloys containing chromium and carbon. Typically these alloys contain from about 11.5 to about 18 weight percent chromium and less than about 0.2 weight percent carbon. Unlike ferritic alloys, these alloys can be hardened by heat treatment and exhibit high strength. These alloys can be made moderately hard and wear resistant by strengthening with precipitates, but are generally only resistant to corrosion in mild environments.

Conventional austenitic stainless steels are ferrous based alloys containing moderate additions of chromium but with relatively little carbon. Conventional austenitic grades of stainless steel contain a minimum of 6% nickel. In general, these alloys achieve better corrosion resistance than conventional martensitic grades of stainless steel. However, powder metallurgy produced austenitic stainless steels may be susceptible to fairly severe crevice type corrosion at certain sintered densities. In addition, since conventional austenitic grades of stainless steel are generally soft they have not achieved comparatively, high wear resistance.

FIG. 1 is a micrograph of an exemplary corrosion resistant compacted article. Referring to FIG. 1, corrosion resistant compacted articles are composed of a dual phase microstructure composed of a blend of ferritic and martensite matrices. The dual phase microstructure is characterized by a certain distribution of a ferritic phase in a martensite matrix. A certain amount of retained austenite may also optionally be present in the alloy matrix. The volume fraction of the ferritic phase is typically at least about 1%, but may range from about 1.0% to about 50%. Preferably, compacted articles are composed of from about 1.0 to about 20.0 volume percent of a ferritic phase in a martensite matrix. More preferably, compacted articles are composed of from about 2.0 to about 15.0 volume percent of a ferritic phase in a martensite matrix. Even more preferably, compacted articles are composed of from about 5.0 to about 8.0 volume percent of a ferritic phase in a martensite matrix.

Without being limited by theory, it is believed that the strength of compacted articles is determined by the onset of plastic flow in the soft phase, i.e., the ferritic phase. Thus, adjusting the proportion of each phase results in a concomitant increase or decrease in strength. For example, by increasing the volume fraction of martensite the tensile properties of the compacted article increase. Preferably, a fine grain structure is formed so that both tensile and elongation properties are improved.

A ferritic factor, K_(m), may be used to define the alloy system of corrosion resistant metallurgical powder compositions. This factor accounts for the influence of alloying additives in stabilizing martensite or ferrite microstructures. “Ferritic factor” is defined by the formula:

K _(m)=(Wt. % Cr)−40(Wt. % C+Wt. % N)+4(Wt. % Mo)−5(Wt. % Cu)−4(Wt. % Ni)−20(Wt. % P)+6(Wt. % Si)−2(Wt. % Mn)+8(Wt. % Ti)+2(Wt. % Al)

Generally, corrosion resistant metallurgical powder compositions have a ferritic factor of from about 4 to about 20. In general, a low K_(m) (less than about 6) reflects corrosive resistant compositions with a predominantly martensitic microstructure, whereas a high K_(m) (greater than about 15) is indicative of corrosion resistant compositions with a predominantly ferritic structure. Compositions having ferrite factors between about 6 and about 15 have a mixed microstructure of martensite and ferrite. Preferably, corrosion resistant metallurgical powder composition have a ferritic factor of from about 6 to about 15, and more preferably from about 6 to about 13. Even more preferably, corrosion resistant metallurgical powder composition have a ferritic factor of from about 8 to about 13 and still more preferable from about 8 to about 11.

FIG. 2A shows an exemplary pseudo phase diagram of a corrosion resistant metallurgical powder composition. One may refer to the attendant phase equilibria to predict the microstructure of sintered compacts. For example, the pseudo-phase diagram in FIG. 2A shows the dependence of phase stability on ferrite factor (K_(m))) and temperature. At normal sintering temperatures for corrosion resistant steels, e.g., ˜1260° C. (˜2300° F.), a compact's microstructure consists of a mixture of ferrite and austenite. Upon cooling to room temperature, the austenite transforms to martensite. The proportions of austenite and ferrite can be determined by the lever rule. The addition of other alloying additives to the iron-chromium alloy system broadens the ferrite+austenite region, and makes it difficult to accurately determine the phases present. Empirical methods and tools, other than a composition's ferritic factor, may be used to predict sintered microstructure, include for example, the Schaeffler diagram and the Delong diagram.

Alloying elements that are blended with an iron based powder are in the form of particles that are generally of finer size than the particles of iron based powder with which they are admixed. The alloying powders generally have a particle size distribution such that they have a d₉₀ value of below about 100 microns, preferably below about 75 microns, and more preferably below about 50 microns; and a d₅₀ value of below about 75 microns, preferably below about 50 microns, and more preferably below about 30 microns.

In one embodiment, corrosion resistant metallurgical powder composition are composed of an iron based powder and from about 11.0 to about 13.0 weight percent chromium, from about 0.001 to about 0.03 weight percent carbon, from about 0.2 to about 0.5 weight percent molybdenum, from about 0.2 to about 0.5 weight percent copper, optionally up to about 1.5 weight percent nickel, optionally up to about 0.03 weight percent sulfur, optionally up to about 0.03 weight percent phosphorus, optionally up to about 1.0 weight percent silicon, optionally up to about 0.25 weight percent manganese, optionally up to about 0.05 weight percent titanium, and optionally up to about 0.05 weight percent aluminum.

In another embodiment, corrosion resistant metallurgical powder compositions are composed of an iron based powder and about 11.6 weight percent chromium, about 0.015 weight percent carbon, about 0.22 weight percent molybdenum, about 0.29 weight percent copper, about 1.03 weight percent nickel, about 0.007 weight percent sulfur, about 0.014 weight percent phosphorus, about 0.84 weight percent silicon, and about 0.10 weight percent manganese.

In a preferred embodiment, corrosion resistant metallurgical powder compositions are composed of an iron based powder and about 11.8 weight percent chromium, about 0.03 weight percent carbon, about 0.25 weight percent molybdenum, about 0.30 weight percent copper, about 1.0 weight percent nickel, about 0.9 weight percent silicon, about 0.3 weight percent oxygen, and about 0.20 weight percent manganese. The particle size of this corrosion resistant metallurgical powder is characterized as having a sieve distribution of +150 to −45 (micrometers), or +100 to −325 (U.S. Standard Mesh).

Corrosion resistant metallurgical powder compositions may optionally include conventional lubricants, binders, and additives, which are known to those skilled in the art. Lubricants may be added to reduce the ejection forces required to remove compacted parts from a compaction die cavity. Examples of such lubricants include stearate compounds, such as lithium, zinc, manganese, and calcium stearates, waxes such as ethylene bis-stearamides, polyethylene wax, polyamides, polyolefins, and mixtures of these types of lubricants. Polyamide lubricants include, for example PROMOLD 450, which is commercially available from Morton International of Cincinnati, Ohio. Other conventional solid lubricants that may be utilized include ACRAWAX, which is commercially available from Lonza Corporation, and KENOLUBE, which is commercially available from Hoganas AG of Sweden. Other lubricants include those containing a polyether compound such as is described in U.S. Pat. No. 5,498,276 to Luk, and those useful at higher compaction temperatures described in U.S. Pat. No. 5,368,630 to Luk, in addition to those disclosed in U.S. Pat. No. 5,330,792 to Johnson et al., each of which is incorporated herein in its entirety by reference.

Binding agents may be added, particularly where an additional, separate alloying powder is used, to bond the different components present in the metallurgical powder composition so as to inhibit segregation and to reduce dusting. By “bond” as used herein, it is meant any physical or chemical method that facilitates adhesion of the components of the metallurgical powder composition. Binding agents include, for example, those found in U.S. Pat. No. 4,834,800 to Semel, U.S. Pat. No. 4,483,905 to Engstrom, U.S. Pat. No. 5,298,055 to Semel et. al., U.S. Pat. No. 5,290,336 to Luk, and U.S. Pat. No. 5,368,630 to Luk, the disclosures of which are each hereby incorporated by reference in their entireties. Binding agents also include, for example, polyglycols such as polyethylene glycol or polypropylene glycol; glycerine; polyvinyl alcohol; homopolymers or copolymers of vinyl acetate; cellulosic ester or ether resins; methacrylate polymers or copolymers; alkyd resins; polyurethane resins; polyester resins; or combinations thereof. The binding agent can further be the low melting, solid polymers or waxes, e.g., a polymer or wax having a softening temperature of below 200° C. (390° F.), such as polyesters, polyethylenes, epoxies, urethanes, paraffins, ethylene bisstearamides, and cotton seed waxes, and also polyolefins with weight average molecular weights below 3,000, and hydrogenated vegetable oils that are C₁₄₋₂₄ alkyl moiety triglycerides and derivatives thereof, including hydrogenated derivatives, e.g. cottonseed oil, soybean oil, jojoba oil, and blends thereof, as described in WO 99/20689, published Apr. 29, 1999, which is incorporated by reference in its entirety. These binding agents can be applied by the dry bonding techniques discussed in that application and in the general amounts set forth above for binding agents. Further binding agents that can be used in the present invention are polyvinyl pyrrolidone as disclosed in U.S. Pat. No. 5,069,714, which is incorporated herein in its entirety by reference, or tall oil esters.

Corrosion resistant powders may be formed into a variety of product shapes known to those skilled in the art, such as for example, the formation of billets, bars, rods, wire, strips, plates, or sheet using conventional practices. These powders are useful in a wide range of practical applications which require an alloy having a good combination of corrosion resistance, strength, and hardness. In particular, the alloy of the present invention can be used to produce structural members and fasteners for automobiles and aircraft and is also well suited for use in medical or dental instruments.

Corrosion resistant compacted articles are prepared by compacting corrosion resistant metallurgical powder compositions. Corrosion resistant metallurgical powder compositions are compacted using conventional techniques known to those skilled in the art. Generally, corrosion resistant metallurgical powder compositions are compacted at more than about 5 tons per square inch (tsi). Preferably, corrosion resistant metallurgical powder compositions are compacted at from about 5 to about 200 tsi, and more preferably, from about 30 to about 60 tsi.

Compacted articles are sintered to form a dual phase microstructure. The compacted part is sintered for a time sufficient to achieve metallurgical bonding and alloying using conventional sintering equipment known to those skilled in the art. The unique dual phase microstructure of corrosion resistant compacted articles is dictated by the selected alloy system of the corrosion resistant metallurgical powder composition and the thermal sintering profile. Preferably, sintering temperature is selected with reference to the ferritic factor of the corrosion resistant metallurgical powder composition. Sintering temperatures are identified for a given compacted article so that the microstructure of the sintered part exhibits a dual phase microstructure.

Sintering temperature is selected with reference to a phase diagram of the alloy system of the corrosion resistant metallurgical powder composition, such as the exemplary pseudo phase diagrams shown in FIG. 2B. With reference to FIG. 2B, sintering temperature is selected so that the intersection point of a vertical line denoting a given ferritic factor and a horizontal line denoting a sintering temperature lies within the “hatched” dual phase region, y+a, marked “A”. The combination of ferritic factor and sintering temperature determine the percentage of both ferrite and high temperature austenite which forms martensite upon cooling. For example, a corrosion resistant metallurgical powder composition having K_(m) of 11 would not be sintered above 2400° F. because the alloy system would form a microstructure consisting of single phase ferrite, i.e., as shown in the phase diagram of FIG. 2B the microstructure would be within the single phase region, α.

Preferably, compacted parts are sintered in a hydrogen atmosphere. A sintering atmosphere may also be composed of a mixture of hydrogen and nitrogen, however the nitrogen in the sintering atmosphere may be absorbed by the compacted article. Adjustment to the ferritic factor of the compacted article should be made as a result of nitrogen addition. Thus, nitrogen can be used as a low cost alloy element to target a desired ferritic factor.

Sintering is advantageously conducted at any temperature that results in sintered part having a dual phase microstructure. Preferably, a sintering temperature of at least 2000° F., preferably at least about 2200° F. (1200° C.), more preferably at least about 2250° F. (1230° C.), and even more preferably at least about 2300° F. (1260° C.). The sintering operation can also be conducted at lower temperatures, such as at least 2100° F.

Additional processes such as forging or other appropriate manufacturing techniques or secondary operations may be used to produce a finished part.

Sintered parts typically have a density of at least about 6.6 g/cm³, preferably at least about 6.68 g/cm³, more preferably at least about 7.0 g/cm³, more preferably from about 7.15 g/cm³ to about 7.38 g/cm³. Still more preferably, sintered parts have a density of at least about 7.4 g/cm³.

Those skilled in the art will appreciate that numerous changes and modifications may be made to the preferred embodiments of the invention and that such changes and modifications may be made without departing from the spirit of the invention. The following examples further describe the metallurgical powder compositions.

EXAMPLES

The present invention is further illustrated by the following examples. It should be understood that these examples, while indicating certain embodiments of the invention, are given by way of illustration only, and should not be construed as limiting the appended claims.

Unless noted otherwise, The densities of green and sintered compacts were determined in accordance with MPIF Standard 42. Tensile testing was performed in accordance with MPIF Standard 10. Impact energy tests were performed in accordance with MPIF Standard 40. Apparent hardness measurements were conducted according to MPIF standard 43. Rotational bending fatigue (RBF) testing was performed pursuant to MPIF Standard 56 wherein rotational speeds ranged from between 7000-8000 rpm at R=−1. RBF tests were conducted using the staircase method to determine the 50% survival limit and the 90% survival limit for 10⁷ cycles. Metallographic specimens of all test compacts were examined by optical spectroscopy in the polished and etched conditions. Etched test compacts were used for micro-indentation hardness testing per MPIF Standard 51.

As used herein, “Pa” means pascal(s), which corresponds to newton(s)/square meter (“N/m²”); “MPa” means megapascal(s); Rb means hardness according to the Rockwell hardness B scale; “g” means gram(s), “cm” means centimeter(s); “UTS” means ultimate tensile strength; and, “YS” means yield strength.

Example 1

Corrosion resistant metallurgical powder compositions, identified as test compositions DP1 through DP5, were prepared and compared with commercially available stainless steel compositions. Test compositions DP1 through DP 5 were prealloyed iron based powders that included the alloying elements shown below in Table 1. The iron and alloy elements of each composition were melt blended and water atomized to provide corrosion resistant metallurgical powder compositions. The atomized powders were than admixed with 0.75 weight percent of lubricant, which was commercially available as Acrawax C, from Lonza Corporation. DP1 though DP5 had ferritic factors ranging from 6.1 to 14.2. These test compositions were compared to two conventional ferritic stainless steel powders, commercially available as 410L and 430L grade powders from Hoeganaes Corporation. The 410L powder was admixed with graphite powder prior to compaction and sintering to provide a higher martensitic microstructure.

TABLE 1 410L + DP1 DP2 DP3 DP4 DP5 graphite 430L Chromium 11.7 11.8 11.6 12.0 12.0 12.5 16.5 Carbon 0.004 0.004 0.005 0.003 0.13 0.011 0.011 Nickel 0.0483 0.0319 0.0445 0.0133 0.0139 0.0246 0.03 Copper 0.27 0.29 0.03 0.48 0.01 0.02 0.02 Molybdenum 0.2 0.2 0.3 0.5 0.3 0.0 0.0 Manganese 0.11 0.13 0.10 0.10 0.11 0.11 0.11 Silicon 0.60 0.85 0.84 0.80 0.81 0.85 0.85 Phosphorous 0.014 0.014 0.014 0.013 0.012 0.014 0.014 Sulfur 0.003 0.014 0.015 0.013 0.006 0.010 0.010 Oxygen 0.04 0.10 0.20 0.24 0.16 0.14 0.26 Nitrogen 0.0483 0.0319 0.0445 0.0133 0.0139 0.0246 0.03 Ferritic Factor, (K_(m)) 6.1 10.2 11.3 13.3 14.2 15.6 19.5

Each powder was uniaxially compacted at 50 tsi (690 MPa) into test pieces. The test pieces were sintered in a hydrogen environment with a dew point of −40° C. (−40° F.) in a continuous belt furnace at 1260° C. (2300° F.) for 45 minutes. The green and sintered density of each compact is shown in Table 2:

TABLE 2 Ferritic Factor Green Density Sintered Density (K_(m)) (g/cm³) (g/cm³) DP1 6.2 6.69 7.03 DP2 10.2 6.68 7.34 DP3 11.3 6.62 7.38 DP4 13.3 6.58 7.35 DP5 14.2 6.59 7.31 410L + 15.6 6.66 7.3 graphite 430L 19.5 6.61 7.16

FIG. 3 is a graph of sintered density verses ferritic factor for exemplary sintered corrosion resistant compacted articles, DP1-DP5, and conventional stainless steel compacts. As shown in Table 2 and FIG. 3, compacts prepared from compositions DP2-DP5 exhibited higher sintered density compared to the convention stainless steel compositions.

Tests were performed to determine the transverse rupture strength (TRS), apparent hardness, yield strength, ultimate tensile strength, and elongation for each compact. FIGS. 4, 5, 6, and 7 show graphs of the these physical properties compared to each compact's ferritic factor. Physical properties are also summarized in Table 3:

TABLE 3 Apparent Ultimate Tensile TRS Hardness Strength 0.20% Offset* Elongation (MPa) (10³ psi) HRB (MPa) (10³ psi) (MPa) (10³ psi) (%) DP1 1578 229 84 687 100 538 78 2.3 DP2 1914 278 88 736 107 564 82 3.1 DP3 1730 251 83 674 98 509 74 4.0 DP4 1201 175 66 444 64 284 41 8.5 DP5 1274 185 68 478 69 297 43 10.0 410L + 1430 208 54 377 55 213 31 16.2 graphite 430L 905 132 54 358 52 202 29 15.3 *“0.20% Offset” is a means of analyzing yield strength. This value refers to the stress required to give 0.20% plastic offset.

As shown in Table 3, compacts prepared from DP1-DP5 exhibited greater ultimate tensile strength and hardness compared to the compacts prepared from conventional compositions. Compacts prepared from DP1-DP3 also exhibited greater transverse rupture strength compared to the compacts prepared from conventional compositions.

Quantitative metallography was performed to measure the actual ferrite content of DP1 through DP5. FIG. 8 is a graph of volume percent of ferrite verses ferritic factor for corrosion resistant compacted articles. Referring to FIG. 8, the volume percent of ferrite in corrosion resistant compacted articles increased as the ferritic factor increased.

As shown in Table 3, corrosion resistant compacted articles exhibited an increase in ductility and a decrease in strength, as the level of ferrite in the microstructure increased. Apparent hardness was directly related to the amount of martensite in the microstructure, and was optimized at Km values between 6 and 11. As shown in FIG. 8, there were minor amounts of ferrite in the microstructure of compacted parts with Km values below 9.

The compacted parts made from DP1-DP5 exhibited a compressibility close to that of the commercially available ferritic steel, 410L. Without being limited by theory it is believed that increasing the alloy content of the powder composition hardened the resulting compacted parts, and therefore had a deleterious effect on compressibility and green density. Although it should be noted that a synergistic effect on sintered density was exhibited by powders prealloyed with nickel, molybdenum and copper. As shown in Table 3, the sintered density of the compacted prepared from corrosion resistant metallurgical powders exceeded 7.3 g/cm³, with the exception of the compact prepared from DP1.

Example 2

The mechanical and physical properties of corrosion resistant compacts were compared to conventional stainless steels typically used in high strength applications. A compacted article, DP6, was prepared according to the steps described above with the composition shown in Table 4 below. The conventional stainless steels tested were commercially available as 17-4PH, 409LNi, and 410L with graphite addition, each sold by Hoeganaes Corporation. The 17-4PH powder was a precipitation hardening, martensitic stainless steel that combines high strength and hardness with corrosion resistance. Upon compaction, articles prepared from this powder were age hardened prior to testing. The 409LNi powder is an

admixture of conventional 409Cb powder commercially available from Hoeganaes Corporation and elemental nickel powder commercially available from Inco-123. 409Cb is typically used in conventional automotive exhaust flange applications. As described above, the 410L+graphite compositions, had a martinsitic microstructure. The compositions of these materials are summarized below in Table 4:

TABLE 4 410L + DP6 graphite 17-4PH 409LNi Chromium 11.6 12.5 17.0 11.3 Carbon 0.015 0.100 0.018 0.013 Nickel 1.03 0.09 4.00 1.30 Copper 0.29 0.08 3.55 0.04 Molybdenum 0.22 0.02 0.03 0.05 Manganese 0.10 0.15 0.15 0.12 Silicon 0.84 0.85 0.85 1.00 Phosphorous 0.014 0.013 0.025 0.01 Sulfur 0.007 0.008 0.010 0.004 Oxygen 0.21 0.30 0.30 0.19 Nitrogen 0.0090 0.0180 0.0200 0.0160 Niobium — — 0.25 0.56 (Columbium)

Each powder was admixed with 0.75 weight percent Acrawax C and uniaxially compacted at 50 tsi (690 MPa) into test parts. The test parts were sintered in a hydrogen environment with a dew point of −40° C. (−40° F.) in a continuous belt furnace at 1260° C. (2300° F.) for 45 minutes. The 17-4PH powder was heat treated at 482° C. (900° F.) for 30 minutes. The static mechanical properties of the test compositions are shown in Table 5:

TABLE 5 Transverse Ultimate Tensile Rupture Strength D.C. Strength 0.20% Offset Elong. (MPa) (10³ psi) (%) (MPa) (10³ psi) (MPa) (10³ psi) (%) DP6 1641 238 −2.99 819 119 612 89 2.5 410L + 1248 181 −2.17 633 92 358 52 4.9 graphite 17-4PH 1421 206 −1.80 757 110 627 91 1.4 409LNi 1255 182 −2.10 612 89 482 70 1.8

As shown in Table 5, DP6 exhibited superior strength and apparent hardness compared to the compacted articles prepared from conventional compositions. The compacted part prepared from DP6 exhibited higher sintered density compared to the other compacts. It was observed that, compared to DP6 compacts, the compacts prepared from 409LNi required longer processing times at high temperature to achieve sufficient diffusion of admixed nickel.

The dynamic properties of the test compositions are shown in Table 6:

TABLE 6 Sintered Impact Fatigue Strength Apparent Density Energy (90% survival) Hardness (g/cm³) (J) (ft · lbf) (MPa) (10³ psi) (HRB) DP6 7.15 65 48 315 45.6 91 410L + 7.05 39 29 330 47.8 79 graphite 17-4PH 6.67 26 19 244 35.4 86 409LNi 7.01 32 24 249 36.0 79

As shown in Table 6, compacts prepared from DP6 outperformed compacts prepared from conventional compositions. Although the 410L composition exhibited favorable compressibility and therefore higher sintered density compared to the other conventional compositions, tight furnace control was necessary to achieve consistent carbon concentrations. Moreover, it was observed that the additions of graphite reduced the corrosion resistance of the final compacted article.

FIG. 9 is a graph of fatigue endurance limits (KSI) verses tensile strength (KSI) for corrosion resistant compacted articles. Referring to FIG. 9, the fatigue endurance of compacts prepared from DP6 was compared to conventional compacted articles. It is commonly known in powder metallurgy applications that resistance to crack initiation, i.e., fatigue endurance, increases as tensile strength increases. As shown in FIG. 9, compacts prepared from DP6 exhibited higher sintered density and tensile strength compared to conventional compacted articles. Accordingly, compacts prepared from DP6 exhibited higher fatigue resistance compared to conventional compacted articles.

Micro-indentation hardness measurements of martensite were measured for each component. The results are shown below in Table 7:

TABLE 7 Micro-indentation Hardness DP6 318 HV 50 gf 410L + graphite 372 HV 50 gf 409LNi 250 HV 50 gf 17-4PH 245 HV 50 gf

As shown in Tables 6 & 7, the combination of strong martensite and ductile ferrite in the compact prepared from DP6 imparts high strength, superior ductility, and superior impact toughness compared to the conventional stainless steels. In addition, without being limited by theory, it is believed that the addition of copper and nickel lead to harder martensite. Micrographs of the compacted parts are shown in FIGS. 10-13. FIG. 10 is a micrograph of a conventional stainless steel compact prepared from commercially available 410L grade powder with graphite addition. FIG. 11 is a micrograph of a conventional stainless steel compact prepared from commercially available 17-4PH grade powder. FIG. 12 is a micrograph of a conventional stainless steel compact prepared from commercially available 409LNi grade powder. FIG. 13 is a micrograph of DP6. For comparative purposes one may refer to FIG. 14, which is a micrograph of a conventional rolled plate wrought stainless steel. As shown in FIGS. 13 and 14, the corrosion resistant compact, DP6, did not exhibit the traditional banded microstructure associated with a rolled plate wrought steel.

Example 3

The corrosion resistance properties of the compacted parts from Example 2 were tested. Two types of corrosion tests were performed. The first corrosion test, a salt spray test, was performed on the compacts according to ASTM Standard B 117-03. Five test bars were tested per alloy. The percent area of the bars covered by red rust was recorded over time. The results are shown below in Table 8:

TABLE 8 Red Rust After 24 Hours (%) DP6 5.00 410L + graphite 84.00 409LNi 20.00 17-4PH 1.0

As shown in Table 8, the high alloy content of the compact prepared from 17-4PH result in the lowest amount of rust after 24 hours, i.e., the highest resistance to corrosion. However, at lower chromium levels the compacts prepared with DP6 exhibited comparable resistance to corrosion. The corrosion resistance of the compact prepared from DP6 was superior to the corrosion resistance of the compacts prepared from 409LNi and 17-4PH respectively. Without being limited by theory it is believed that the addition of minor amounts of copper and molybdenum lead to improvement in corrosion resistance.

The second type of corrosion tests measured the oxidation that occurred on sintered cross-sections of test bars during cycling from room temperature to 1200° C. (2192° F.) in air. This test was designed to mimic performance in high-temperature oxidation conditions, such as for example, exhaust flange applications. Each heating cycle lasted 2 to 4 hours, and was repeated as many as 400 times. Changes in mass were recorded at each interval by a thermal gravimetric (TG) unit. The mass gain of each compact is shown in FIG. 15. FIG. 15 is an X-Y graph of the percentage of mass gain verses number of oxidation cycles for corrosion resistant compacts and conventional stainless steel compacts. As shown in FIG. 15, each compact exhibits an initial high rate oxidation until oxygen saturation was achieved. Although the compact prepared from 17-4PH exhibited the lowest initial mass gain and lowest oxygenation rate, the compact prepared from DP6 saturated at a lower number of cycles. The compact prepared from DP6 exhibited a lower oxidation rate compared to the compacts prepared from 409LNi and 410L with graphite addition.

Example 4

The mechanical and physical properties of another corrosion resistant compacts were analyzed. A compacted article, DP7, was prepared composed of an iron based powder and about 11.8 weight percent chromium, about 0.03 weight percent carbon, about 0.25 weight percent molybdenum, about 0.30 weight percent copper, about 1.0 weight percent nickel, about 0.9 weight percent silicon, about 0.3 weight percent oxygen, and about 0.20 weight percent manganese. The corrosion resistant metallurgical powder was characterized as having a sieve distribution of +150 to −45 (micrometers), or +100 to −325 (U.S. Standard Mesh).

FIGS. 16-25 show graphs of physical properties of DP7. FIGS. 26 and 27 show micrographs of DP7. Referring to FIGS. 26 and 27, the corrosion resistant compact exhibited a dual phase microstructure of ferrite and martensite. As shown in FIGS. 16-25, DP7 had high strength and hardness with good ductility and very good impact properties.

Thus as shown in Examples 1-4, corrosion resistant compacts prepared from the present compositions and methods, yield similar or better corrosion resistance without the need for costly, high alloy content, such as for example, high chromium content. These corrosion resistant compacts exhibit an excellent combination of mechanical properties without the need for secondary processing techniques, such as for example, heat treatments. This combination of features make the present corrosion resistant compacts an improved, cost-effective solution for applications requiring high strength and corrosion resistance. 

1. A method of preparing corrosion resistant compacted articles comprising the steps of: (a) providing a metallurgical powder composition comprising: (i) as a major component, an iron-based powder, (ii) as a minor component, alloy additives comprising: about 5.0 to about 30.0 weight percent chromium, and about 0.001 to about 0.1 weight percent carbon, (b) compacting the metallurgical powder composition in a die at a pressure of at least 5 tsi; wherein the metallurgical powder composition achieves a dual phase microstructure of martensite and ferrite upon sintering.
 2. The method of preparing corrosion resistant compacted articles of claim 1 wherein the metallurgical powder composition further comprises molybdenum, copper, nickel, sulfur, phosphorus, silicon, manganese, titanium, aluminum, or combinations thereof.
 3. The method of preparing corrosion resistant compacted articles of claim 1 wherein the metallurgical powder composition comprises a prealloyed powder.
 4. The method of preparing corrosion resistant compacted articles of claim 1 wherein the ferrite factor of the metallurgical powder composition is from about 6 to about 20 and ferrite factor (K_(m)) is defined by the formula: K _(m)=(weight percent Cr)−40(weight percent C+weight percent N)+4(weight percent Mo)−5(weight percent Cu)−4(weight percent Ni)−20(weight percent P)+6(weight percent Si)−2(weight percent Mn)+8(weight percent Ti)+2(weight percent Al).
 5. The method of preparing corrosion resistant compacted articles of claim 1 wherein the iron based powder is a water-atomization powder.
 6. The method of preparing corrosion resistant compacted articles of claim 1 further comprising the step of sintering the compacted article to form a dual phase microstructure of martensite and ferrite.
 7. The method of preparing corrosion resistant compacted articles of claim 1 wherein the compacted article exhibits a density of greater than about 6.6 g/cm³.
 8. The method of preparing corrosion resistant compacted articles of claim 6 wherein the sintered compact comprises from about 1.0 to about 20.0 volume percent of ferrite in a martensite matrix.
 9. The method of preparing corrosion resistant compacted articles of claim 6 wherein the sintered compact exhibits a sintered density of from about 7.15 g/cm³ to about 7.38 g/cm³. 